The invention relates to precision radar range measurements of an object, or objects, (such as a tennis ball or baseball in play hereinafter referred to as the "object") based on the phase of the radar return from the "object", with measurements made at multiple frequencies, the Chinese Remainder Theorem being used to remove the range ambiguity inherent in using the phase as a measurement of range, and the radar return of the "object" of interest being separated from the radar return of competing clutter.
For fifteen years or more, the professional tennis community has been seeking a device to automatically call the lines in tennis matches, thus replacing up to eleven line umpires (including the net judge) per match and reducing the number of altercations during a match. Many devices have been built but most, if not all, have had some failing or limitation which made them unacceptable. It is highly probable that radar was considered by many of those attempting to find a solution. A radar solution probably was not given serious consideration because standard radar technology is incapable of achieving the required accuracy. The present invention removes this limitation.
For baseball, at least one attempt has been made to provide electronic tracking of a pitched ball. U.S. Pat. No. 4,545,576 discloses such a device using video cameras. To date, while partially successful, it has not proven sufficiently accurate to call balls and strikes (See UNIX Today! Mar. 18, 1991).
Standard radar techniques use the time of arrival of the return from an "object" as the basis of a range measurement. One-foot accuracy with such techniques is considered to be excellent. Also with such standard techniques, the radar-return's pulse-to-pulse phase difference is used for Doppler frequency measurements, but not to improve range measurement accuracy. However, as far as it is known, the phase of the return in conjunction with multiple transmitter frequencies and the Chinese Remainder Theorem has never before been used to make precision measurements of the range to an "object" (such as a tennis ball or baseball in flight).
The Chinese Remainder Theorem is presently used in some radar systems to resolve range ambiguities. But again, the range measurements are based on the time of arrival of the radar-return. In addition, the radar's pulse-repetition frequency (not the transmitted frequency as proposed herein) is the parameter which is varied to provide the necessary information for the Chinese Remainder Theorem.
In the field of laser range finders, the use of multiple laser modulating frequencies, phase detection and the Chinese Remainder Theorem has been proposed in U.S. Pat. No. 4,537,502. This prior art also mentions direct use of multiple frequencies instead of modulating a laser beam. This prior art specifically requires a "counting frequency f.sub.o " such that the multiple frequencies are related by: EQU f.sub.o =N.sub.r .multidot.N.sub.1 .multidot.f.sub.1 =N.sub.r .multidot.N.sub.2 .multidot.f.sub.2 =. . .
This prior art relates the accuracy of the technique to the size of N.sub.r and thus to the size of f.sub.o. To obtain the accuracies sought with the current invention, such a counting frequency would be required to be in the order of 200 GigaHertz, which is well beyond the state of the art for the practical use of such a counting frequency. The need for such a counting frequency and the resulting restrictions is considered to be unique to the proposed instrumentation of the prior art and does not apply to the present invention.